(1) Field of the Invention
The present invention relates to a magnet compound material to be compression molded, which is used for producing molded elongate magnet to be buried in magnet rollers employed in image-forming apparatuses such as copiers, facsimile apparatuses and printers. The invention also relates to such molded elongate magnet produced from the magnet compound material, magnet rollers in which such molded elongate magnet are buried, developing agent-carrying bodies having such magnet rollers, a developing apparatus having such a developing agent-carrying body, a processing cartridge having such a developing apparatus, and an image-forming apparatus having such a processing cartridge. The term “elongate” means that a longitudinal length of the elongate magnet is considerably larger than a longitudinal length of a sectional view of the magnet as cut in a direction orthogonal to the longitudinal direction of the elongate magnet.
(2) Related Art Statement
“A high-performance developing apparatus, which develops latent images formed on an image-carrying body with use of a two-component developing agent composed of a toner and an magnetic grains” (hereinafter referred to “SLIC developing apparatus” (SLIC: Sharp Line Contact), have recently attracted public attention, and solved problems in images. A developing agent-carrying body (developing roller) to be mounted on this SLIC developing apparatus is required to meet the following characteristics: (1) a half-value width of a developing pole is not more than 20° (about 50° in the conventional two-component development) and (2) the magnetic flux density is in a range of 100 to 130 mT (80 to 120 mT in the conventional two-component development). In the SLIC developing apparatus, it is necessary that the magnetic flux density of the developing pole is increased and the half-value width is reduced to not more than ½ of that in the conventional developing pole. However, according to the conventional ferrite-based magnet, decrease in the half-value width lowers the magnetic flux density. Thus, both of (1) and (2) cannot be unfavorably satisfied. The SLIC developing apparatus used herein is intended to moan that the developing apparatus includes a developer carrier made up of a nonmagnetic sleeve and a magnet roller fixed in place within said nonmagnetic sleeve and having a magnet for scooping up a developer, a magnetic pole for conveying said developer and a main magnetic pole for causing said developer to rise in a form of a head, a flux density in a direction normal to said main magnetic pole has an attenuation ratio of 40% or above. See U.S. Pat. No. 6,385,423 B1.
The specifications of the developing agent-carrying bodies used in the SLIC developing apparatuses depend upon kinds of the apparatuses, diameters of the rollers, etc. In recent apparatuses, the magnetic flux density is required to have 100˜130 mT for a developing pole and an adjacent pole thereto, and high magnetization is largely demanded. The range of 100 to 130 mT in terms of the magnetic flux density on the developing agent-carrying body is converted to a range of 13 to 16 MGOe in terms of (BH) max value. Therefore, it is demanded that the magnetic flux density is not less than 13 MGOe, that is, a high magnetism magnet which exhibits not less than 100 mT when measured at a gap of 1 mm from a surface of a magnet in which a magnet body is attached to a non-magnetic body is sought.
Sm.Co based, Nd.Fe.B based and Sm.Fe.N based rare earth magnetic materials are well known as magnetic materials having high energy products for the magnetic bodies. However, since the Sm.Co based rare earth magnetic material has high material cost, it has been hardly used in general. Recently, Nd.Fe.B based magnetic material and the Sm.Fe.N based magnetic material have been frequently used. In order to obtain magnets having arbitrary shapes, a synthetic resin composition containing such a magnetic powder needs to be kneaded and molded in a desired arbitrary shape.
Conventionally, plastic magnets having arbitrary shapes have been used by molding the mixed material in which the magnetic material is kneaded with a plastic resin material. Such plastic magnets are produced by either one of the following methods: (1) injection molding (JP-2002-190421-A2), (2) extrusion molding (JP 2001-93724-A2), and (3) compression-molding (JP2001-118718-A2).
According to the above injection molding method (1), the mixed composition is melted under heating to have sufficient flowability, and a predetermined shape is given by injecting the heat-melted material into a mold. According to the above extrusion molding method (2), the mixed composition is melted under heating, and a predetermined shape is given by extruding the heat-melted material from a mold and solidifying it under cooling. According to the above compression-molding method (3), the mixed composition is charged into a mold where it is compression molded.
In the above injection molding method (1), since the dimension of the molded product is determined by the dimension of the mold, a magnet having a strange shape can be molded at a highly dimensional precision. However, a compounding ratio of the binder resin needs to be increased to smoothly flow the mixed composition into the mold, so that the compounding ratio of the magnet material must be decreased. Thus, it is unfavorably difficult to obtain magnets having high magnetism.
In the above extrusion molding method (2), since the mixed composition is continuously molded, productivity is high. To the contrary, it is unfavorably difficult to realize highly dimensional precision as compared with the injection molding method. Further, it is also difficult to increase the compounding ratio of the magnet material like the injection molding method. Consequently, it is also difficult to obtain magnets having high magnetism.
In the above compression-molding method (3), since the compounding ratio of the binder resin can be decreased, the density of the magnetic powder can be increased. Thus, this molding method is suitable for molding small-size magnets having high magnetism. However, in the compression-molding method (3), the pressing pressure needs to be increased to mold a large-size magnet having high magnetism so that the density of the molded product may be increased. At present, when the ordinary epoxy compound as the compression-molding compound is used, not less than 100 kN/cm2 is required for the pressing pressure. Consequently, a 1000 kN/cm2 class pressing machine is required to produce a molded elongate magnet product having a specific pole in magnet roller. Therefore, the construction of the compression-molding apparatus becomes large. Further, since the mechanical strength of the mold needs to be increased, it is unfavorably difficult to produce elongate magnets by compression-molding in a commercial level.
Some magnetic materials are isotropic, and other are anisotropic. Higher magnetism can be realized for magnetic materials having anisotropic property in which a magnetizing axis can be more easily aligned by applying a magnetic field thereto. An Nd.Fe.B based magnetic material treated with hydrogen at high temperature and having high anisotropy is proposed as the same kind of the currently practically used rare earth magnetic material having high magnetism (JP 10-135017-A2 and JP 8-31677-A2). Molded rare earth-based magnetic powders, which are produced by injection molding or extrusion molding with use of a magnet compound material containing Nd.Fe.B based magnetic powder, are commercially available as the molded rare earth-based magnetic bodies. The magnetism of such molded products is 6 to 9 MGOe in terms of (BH) max value, which is not sufficient.
In order to produce magnets having high magnetism of not less than 13 MGOe, the present inventors investigated use of the anisotropic Nd.Fe.B based magnetic material now having the highest magnetism, but they found out that the magnetism of the anisotropic Nd.Fe.B based magnetic material was 10 to 12 MGOe at most in terms of the (BH) max value at present when it was produced by the injection molding or the extrusion molding.
In general, the epoxy based thermosetting resin is used as the binder resin in the compound to be compression-molding. The epoxy resin and a curing agent are compounded in a entire amount of 1 to 10 wt % into the magnet material, and a dry compound is obtained in which the epoxy resin/curing agent is attached around the magnet material. However, in order to use the epoxy resin in the compound in a dry state, it is necessary to use solid epoxy resin and solid curing agent. Many materials such as aromatic amine-based, dicyandiamide-based and imidazole-based materials are available as the solid curing agent. Since any of these materials has a high curing temperature, the curing temperature needs to be at least 150° C. and the curing time is long and needs to be not less than 60 minutes.
The magnetic materials have such a property that their magnetisms is reduced with heat. Particularly since the anisotropic Nd magnet material is likely to decrease its magnetism with heat. Therefore, the magnetic characteristic (BH) max is unfavorably decreased by about 15% in the beat treatment of 150° C. and 60 minutes. Therefore, the thermosetting epoxy resin cannot be practically used as the binder resin. Even if a resin composition composed mainly of a thermoplastic resin is used as the binder resin, its magnetism cannot be prevented from being decreased with heat. Under the circumstances, when a kneaded compound composed mainly of a thermoplastic resin obtained by grinding and classifying and having a low softening point is used as the binder resin to suppress decrease in magnetism with heat, binder resin particles obtained by grinding and classifying have unstable particle shapes and distribution, so that sufficient molded density and magnetic flux density cannot be obtained. For this reason, there is a limit that the magnetic flux density of around 70 mT can be obtained on the average among lots. In addition, variations in the magnetic flux density are as much as around 20 mT among the lots of the binder resin particles.
When a kneaded material composed mainly of a thermoplastic resin having spherical particle shapes with a low softening point is used as the binder resin, mold-filling property is increased to raise the molded density and thereby enhance the magnetic flux density. The magnetic flux density of the thus molded magnet is around 95 mT, and variations in the magnetic flux density are as much as around 12 mT among the lots of the binder resin particles. Variations owing to the lots of the binder resin particles can be adjusted by varying magnetizing voltage. However, it takes a long time to adjust the magnetism, and if the magnetizing voltage is lowered, the magnetic flux density at opposite end portions of the magnet is unlikely to be decreased. Thus, since deviations in the magnetic flux density become larger in the axial direction of the magnet, there is a problem that the magnet having a uniform magnetic flux density cannot be obtained.
Since a compound is filled inside a mold cavity having a constant volume according to compression-molding method in a magnetic filed, the filled density differs depending upon the particle diameter distribution of the binder resin particles. FIG. 12 is a schematic view of the conventional magnet compound material to be used in the compression-molding method. When the magnetic powder 201 of the magnet compound material is mixed with the binder resin 202, the magnetic powder 201 and the binder resin particles 202 are charged plus and minus, respectively through friction electrification, and the binder resin particles 202 are electrostatically attached to around the magnetic powder 201. However, since the electrostatically attaching force of the binder resin particles 202 is relatively small, the binder resin particles are likely to be detached from the magnetic powder. Accordingly, as shown in FIG. 12, there appear binder resin-rich layers and magnetic powder-rich layers, so that variations in magnetic flux density (magnetic force) become greater in the magnet molded from the magnet compound material. Further, since the particle diameter distribution differs among the lots of the binder resin particles, variations in the magnetic flux density (magnetism) increase. In this way, when there are formed the binder resin particle-rich layers and the magnetic powder rich-layers or the particle diameter distribution of the binder resin particles 202 differs depending upon the lots, the filled density inside the mold varies. Thus, the molded density and the magnetic force vary among the magnets. However, when the magnet is used as a magnet in a developing agent-carrying body, an elongate magnet of around 300 mm in length is necessary, so that variations in magnetism of the magnetic pole need to be suppressed to within ±3 mT.